1. Field of the Invention
The present invention relates to a technique of reducing influences of acoustic signals in electroencephalogram measurement.
2. Description of the Related Art
In recent years, due to decreases in the size and weight of devices, wearable devices such as stereo headphone-type music players and head-mount displays (hereinafter also referred to as “HMDs”) are gaining prevalence.
Usually, as an interface with which to manipulate a device, physical input manipulations using an input device have been employed, e.g., “pressing a button”, “moving a cursor and making a confirmation”, or “manipulating a mouse while watching a screen”. However, if the aforementioned physical input manipulations are required when manipulating a device whose main body has a small size and which is characterized to be handsfree, e.g., an HMD, the handsfree feature will be undermined, thus being ineffective. Furthermore, in the case of manipulating a very small device such as a hearing aid, the operation buttons and/or dials will become very small, which will be inconvenient for frequent manipulations.
Therefore, an interface which utilizes an electroencephalogram, such that it can be easily manipulated by a mere thought of a user, will be convenient because physical manipulations are not required. What will be even more convenient to have is an ability to monitor the state (e.g., emotions) of a user by utilizing an electroencephalogram or the like of the user, and automatically control the mode, etc., of a device in accordance with the monitored state.
As used herein, an “electroencephalogram” refers to the electrical activities of cranial nerve cells (encephalic activities) which are measured based on a difference between the potential of a reference electrode which is placed on the head of a human and the potential of a measurement electrode, for example. It is known that an electroencephalogram represents encephalic activities.
In recent years, development efforts have been directed toward techniques which utilize an electroencephalogram. Japanese Laid-Open Patent Publication No. 2005-34620 discloses an interface which utilizes an electroencephalogram. This publication discloses a technique of determining an option which a user wishes to select, by using a characteristic signal of the electroencephalogram called an event-related potential. Japanese Laid-Open Patent Publication No. 7-204168 discloses a technique which determines an emotional state based on an electroencephalogram and renders it into a numerical representation.
Conventionally, an electroencephalogram has been measured with electrodes being worn according to the position notation of the International 10-20 system. For example, the measurement electrode has been worn at the parietal. In Japanese Laid-Open Patent Publication No. 2005-34620, supra, electroencephalogram measurements are taken at the Pz and Cz positions according to the International 10-20 system. In Japanese Laid-Open Patent Publication No. 7-204168, supra, electroencephalogram measurements are taken at positions centered around the occiput in the neighborhood of Pz. This publication utilizes α waves, i.e., electroencephalograms in a band from 8 to 13 Hz, which are known to be recordable with a relatively large amplitude.
However, having to wear any electrodes for electroencephalogram measurement at the parietal or the occiput, other than at the portions where a wearable device (e.g., an HMO, a music player, and a hearing aid; the same notion will similarly apply hereinafter) would come in contact with the body, will present a burden on the user engaging or disengaging it, and cause discomfort or the like due to the fact that they are always worn on the scalp.
Therefore, it is necessary to accommodate the electrodes for electroencephalogram measurement within the range which is occupied by a shape that is usually needed by conventional wearable devices, so that the electrode placement is completed as soon as the wearable device is worn, thus reducing the burden of the user.
FIG. 1 shows an example of headphones 1001 on which electrodes 1000 for electroencephalogram measurement are provided, according to Japanese Laid-Open Patent Publication No. 2001-187034. In this publication, the electrodes 1000 for electroencephalogram measurement are provided on a support band portion of the headphones 1001, thus resulting in a shape such that the electrodes for electroencephalogram measurement will come in contact with the scalp as soon as the headphones 1001 is worn.
The potential of an electroencephalogram which is measurable on the scalp is very weak, e.g., on the order of 10 μV to 100 μV. Therefore, the electroencephalogram is susceptible to the influences of electrical noises that are generated by neighboring devices. When electrodes are provided in the vicinity of audio signal generators (electro-acoustic transducers) as in Japanese Laid-Open Patent Publication No. 2001-187034, supra, electrical noises from the electro-acoustic transducers will be superposed on the measured electroencephalogram. On the other hand, Japanese Laid-Open Patent Publication No. 9-131331 discloses removing noises in a band from 4 to 30 Hz of the electroencephalogram, by using a preestimated noise spectrum shape.
However, it has hitherto been considered that the frequency band of the audible range is not a noise source. The reason is a belief that the frequency band of an electroencephalogram for use does not overlap the audible range. The frequency band of an acoustic signal which is output from an audio-visual device spans the whole or a part of the frequency band of the human audible range, i.e., 20 Hz to 20000 Hz. The frequency band of electrical noises ascribable to the electro-acoustic transducers which output acoustic signals extends beyond the lower frequency limit (about 20 Hz) of acoustic signals. On the other hand, the frequency band of the electroencephalogram for use contains event-related potentials at 10 Hz or less, θ waves from 4 Hz to 8 Hz, and α waves from 8 Hz to 13 Hz, for example. In other words, the acoustic signal frequency band differs from the electroencephalogram frequency band. Also for the β waves at 13 Hz or higher which are contained in the electroencephalogram, it is often the case that a low-pass filter with a cutoff frequency of about 100 Hz is used at measurement. Thus, it has been believed that most of the electrical noises ascribable to acoustic signals in the audible range are not a major problem to electroencephalogram measurement.